As mentioned herein, everywhere where the context so permits, the reference to a “gas” or “gases” respectively includes “vapor” or “vapors” The fabrication of semiconductor devices often requires the careful synchronization and precisely measured delivery of as many as a dozen or more gases to a process tool, such as a process chamber or reactor. Various recipes are used in the manufacturing process, and many discrete processing steps can be required. For example, a semiconductor device may be required to be cleaned, polished, oxidized, masked, etched, doped, metalized, etc. The steps used, their particular sequence, and the materials involved all contribute to the making of a particular device.
As device sizes continue to shrink below 90 nm, the semiconductor roadmap suggests that atomic layer deposition (ALD) processes will be required for a variety of applications, such as the deposition of barriers for copper interconnects, the creation of tungsten nucleation layers, and the production of highly conducting dielectrics to name just a few. In the ALD process, two or more precursor gases sequentially flow over a wafer surface in a process chamber maintained under vacuum. The two or more precursor gases usually are introduced in a series of successive pulses into one or more reactors, so that the gases can react with the sites or functional groups on the wafer surface. The pulses need to be carefully controlled so that the number of moles of a gas delivered is precise. In fact, with an ALD process the control usually needs to be so precise as to control the number of atoms or molecules of a gas delivered in each pulse. See, for example, U.S. Pat. Nos. 7,615,120 (Shajii et al.); 6,913,031 (Nowata et al.) and 6,887,521 (Basceri); and US Patent Application Publication Nos. 2007/0022951 (Spartz) and 2006/0130755 (Clark). When all of the available sites are saturated with one of the precursor gases (e.g., gas A), the reaction stops and a purge gas is typically used to purge the excess precursor molecules from the process chamber. The process is typically repeated, as the next precursor gas (e.g., gas B) flows over the wafer surface. A typical cycle for a simple process using only two precursor gases is defined, for example, as one pulse of precursor gas A, purge, one pulse of precursor gas B, and purge. This sequence is usually repeated until the final thickness is reached. Each of these cycles of self-limiting surface reaction with precursor gases results in one mono-atomic layer of deposited film per cycle.
The pulses of precursor gases introduced to a tool, such as a processing chamber or reactor are normally controlled using on/off-type or shut-off valves. One valve is used as an inlet valve to the reservoir to be charged, while a second is used as an outlet valve from the reservoir to control the pulse delivered to a tool. The outlet valve is simply opened for a predetermined period of time necessary to deliver desired molar amount of precursor gas from the storage reservoir. One current method of controlling the flow of pulses, exemplified in the disclosure of U.S. Pat. No. 7,615,120 (Shajii et al.), includes a technique of controlling the timing of the opening and closing of the appropriate inlet valve for charging a storage reservoir. The number of moles delivered is based on the pressure drop in the storage reservoir of known volume upstream of the outlet valve and a real time gas temperature model to address temperature fluctuations of the gas in the storage reservoir volume due to transitions from valve operations, i.e., opening and closing of the inlet and outlet valves used to control the flow of gas or vapor into and out of the reservoir. This approach requires prior knowledge of the gas since it is dependent on the properties of the gas flowing through the system.